In devices such as that shown in Ser. No. 347,071 now U.S. Pat. No. 4,510,847, piston and cylinder drive mechanism is mechanically coupled to a movable arm such as the hoisting device on a tractor or the like. A hydraulic pressure medium is supplied to the piston and cylinder assembly to cause it to move in one direction, and the piston and cylinder assembly can be vented to allow it to move in the other direction. The application of pressure or the venting of the supply conduit is controlled by the position of a multiple position valve which is acted upon by fluid pressure operated control devices. The control devices are connected to adjustable throttle valves, two of these valves being moved to define an input or reference signal and the others being moved in response to the actual position and the amount of force being applied to the adjustable arm.
Two of these adjustable throttle valves are connected consecutively, i.e., in series with the junction between them being coupled to one of the control devices. The other two are similarly connected to the other control device and also to a pressure ratio valve which is connected to this control device through an additional adjustable throttle. Because of this connection, both control mechanisms of the movable valve are influenced by each change of the adjustable throttles.